1. Field of the Invention
The present invention relates to a semiconductor device including a circuit formed by using a thin film transistor (hereinafter also referred to as a TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic device provided with, as a component, an electrooptic device typified by a liquid crystal display panel or a light-emitting display device having an organic light-emitting element.
In this specification, a semiconductor device generally refers to a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (several nm to several hundred nm) formed over a substrate having an insulating surface has attracted attention. A thin film transistor is broadly applied to electronic devices such as an IC and an electrooptic device, and particularly, a thin film transistor as a switching element of an image display device is expected to be developed quickly.
In particular, an active matrix display device (a liquid crystal display device or a light-emitting display device) provided with a switching element formed of a TFT for each of display pixels arranged in matrix has been actively developed.
In an active matrix display device, development of expanding an effective image region in a pixel portion has been advanced. In order to enlarge an area of an effective image region, it is necessary to make an area of a TFT arranged in a pixel portion (a pixel TFT) as small as possible. In addition, in order to reduce a manufacturing cost, development of forming a driver circuit and a pixel portion over the same substrate has been also advanced. In particular, a TFT using a polysilicon film can be operated at high speed because field effect mobility is higher that that of a TFT using an amorphous silicon film.
In a module mounted in a display device, a pixel portion for performing image display for each function block and a driver circuit for controlling the pixel portion such as a shift register circuit, a level shifter circuit, a buffer circuit, or a sampling circuit, which is based on a CMOS circuit, are formed over one substrate.
In a case where a driver circuit and a pixel portion are formed over the same substrate, there is a tendency that an area of a region except for a pixel region, which is referred to as a frame portion, is increased, as compared with a thin film transistor in which a driver circuit is mounted by a TAB method. In order to reduce an area of a frame portion, it is necessary to make scale of a circuit forming a driver circuit smaller.
In particular, in a light-emitting display device provided with organic light-emitting elements (EL elements) arranged in matrix, a plurality of TFTs having different functions is necessary in one pixel. Further, also in a liquid crystal display device, it is attempted to form a switching TFT and a memory element such as an SRAM in one pixel. In addition, even in the case where a pixel portion and a driver circuit are formed over the same substrate, it is expected to downsize as much as possible.
In Patent Document 1 (Japanese Patent Application Laid-Open No: H10-98189), a field effect transistor having a gate electrode in which one side portion is thin is disclosed. The field effect transistor of Patent Document 1 has a structure in which a cross-sectional shape of the gate electrode is an L-shape, a sidewall spacer is formed along a back side of the L-shape, and a low-concentration drain (LDD: Lightly Doped Drain) is formed only on a drain side by injecting an ion in a high-concentration by using the gate electrode provided with the sidewall spacer as a mask. In addition, a silicide layer is formed over the surface of the L-shape of the gate electrode.
Further, in Patent Document 2 (Japanese Patent Application Laid-Open No: H7-142734), a thin film transistor having an LDD region on one side is disclosed. The thin film transistor of Patent Document 2 is provided with a sidewall spacer only on one side of the gate electrode. It is described in Patent Document 2 that a high-concentration drain region below the sidewall spacer that is formed in ion injection by using the sidewall spacer as a mask has an inclined connection structure, and the inclined connection structure has similar effect to the LDD structure.
Further, in Patent Document 3 (Japanese Patent Application Laid-Open No: H10-32327), a method for manufacturing a semiconductor device is disclosed, in which sidewalls having different widths are formed on a source side and a drain side, and an ion is injected to form a high-concentration impurity diffusion layer.
Further, a process for manufacturing a TFT is disclosed in Patent Document 4 (Japanese Patent Application Laid-Open No: 2002-151523) by the present applicants, in which a photomask or a reticle provided with an assist pattern that is formed of a diffraction grating pattern or a semi-transmitting film and has a function of reducing light intensity is employed in a photolithography step of forming a gate electrode.
Further, a thin film transistor having two gate electrode layers and an LDD region on one side is disclosed by the present applicants in Patent Document 5 (Japanese Patent Application Laid-Open No: 2000-228527).